The invention relates to a process for the electrochemical preparation of chlorine from aqueous solutions of hydrogen chloride in an electrolysis cell.
Aqueous solutions of hydrogen chloride (hydrochloric acids) are obtained, for example, as byproducts in the preparation of organic chlorine compounds by chlorination with elemental chlorine. Many of these organic chlorine compounds are intermediates for the industrial production of plastics. The aqueous hydrogen chloride solutions obtained have to be utilized. They are preferably utilized by preparing chlorine again from the aqueous solutions of hydrogen chloride, which chlorine can then be used, for example, for further chlorinations.
The reaction to chlorine can be effected, for example, by electrolysis of the aqueous solutions of hydrogen chloride at a gas diffusion cathode. A corresponding process is disclosed in U.S. Pat. No. 5,770,035. According to U.S. Pat. No. 5,770,035, the electrolysis is effected in an electrolysis cell having an anode space, with a suitable anode, for example a noble metal-coated or noble metal-doped titanium electrode, which cell is filled with the aqueous solution of hydrogen chloride. The chlorine formed at the anode escapes from the anode space and is fed to a suitable working-up stage. The anode space is separated from a cathode space by a commercial cation exchange membrane. On the cathode side, a gas diffusion electrode rests on the cation exchange membrane. Behind the gas diffusion electrode is a current distributor. An oxygen-containing gas or pure oxygen is usually passed into the cathode space.
The anode space is kept at a higher pressure than the cathode space. As a result, the cation exchange membrane is pressed onto the gas diffusion cathode and this in turn onto the current distributor. The pressure can be adjusted, for example, by means of a liquid seal through which the chlorine gas formed in the anode chamber is passed.
The process disclosed in U.S. Pat. No. 5,770,035 has the disadvantage that a comparatively large amount of hydrogen is formed at the gas diffusion cathode at high current densities, by which in particular current densities greater than 4,000 A/m2 are meant. However, high current densities are necessary for economic reasons when the process is carried out industrially. In addition, a comparatively high voltage is established at high current densities, resulting in high energy consumption.
It is an object of the invention to provide a process for the electrochemical preparation of chlorine from aqueous solutions of hydrogen chloride, as little hydrogen as possible being formed and as low a voltage as possible being established even when working with high current densities.
The invention relates to a A process comprising electrochemically preparing chlorine from an aqueous solution of hydrogen chloride in an electrolysis cell having (a) at least one anode chamber containing an anode, (b) at least one cathode chamber having (i) an oxygen-consuming cathode and (ii) a pressure that is at least about 1.05 bar, and (c) a cation exchange membrane for separating the anode and the cathode chamber, in which an aqueous solution of hydrogen chloride passes into the at least one anode chamber and an oxygen-containing gas passes into the at least one cathode chamber.